Comfortable Cruising Speed

65beam · May 25, 2016

speed

If you want to know what your actual speed at a certain RPM is just turn on the GPS. my Garmin is pretty accurate. another thing to consider is that back when these were fairly new cars it was considered normal to replace the bearings after 30K to 40 K miles. that should be an indication of what consistent high RPM's could do to a stock engine. oil pump wear also comes into play at sometime. the series 4 I've been driving since the 60's spun #3 rod bearing at close to 40K. we didn't have many miles of interstates or even multiple lane roads so they were able to keep up with the flow of traffic. don't expect them to keep up with the Honda civics of today that are running at 80 MPH without some problems.

Tom H · May 25, 2016

Michael, I think you are confusing Hodee with someone else. Hodee says he has a 3.89 axle, large 15" tires, and no OD.

One problem I have with the tire/RPM calculator is that I have measured the rolling circumference of my tires and find it is about 2% smaller than what you would calculate from the tire size. So I use the measured rolling circumference in calculating RPM at speeds.

It's easy to check your tach by calculating the exact RPM at 60 MPH and using a using a GPS to set your speed to 60 MPH. I'd recommend measuring the rolling circumference - just mark with chalk the tire and a point on the driveway where the tire touches the drive, roll the car 2 or 3 tire rotations, and mark again. Measure the distance and divide by 2 or 3. That's the rolling circumference. Multiply 5280 times 12 to get inches in a mile. Divide that by the measured rolling circumference to get wheel revs per mile. Multiply that by the axle ratio (e.g.4.221) to get Engine revs per mile. and that number is exactly the same as the RPM at 60 MPH.

To answer the original question, my Alpine has Chevy rods, and an extra 0.100" stroke, Vizard mod, but original valve seats, 175-70/R 13 tires, 3.89 axle, and OD. I use premium gas (93 or 94). I have put 16,000 miles since engine rebuild and 80% of that mileage was run at 70-74 MPH, 3500-3800 RPM, with no problem. The engine before rebuild burned a valve and broke a piston ring in #4 cyl which I attribute to overheating. during rebuild I made sure to clean the cooling passages, especially around #4 , and I had my radiator re-cored. It runs much cooler now.

Tom

Tom

alpine_64 · May 25, 2016

Tom you are correct.. my post should have been to Jimjordan2. ..

He had stated 65mph = 3800 in his car, with a 4.22 and 185/70/13 tyres.

This would give:
a tyre dia of 23.2"
Circumference of 72.88"
So 20.36 mph per 1000 rpm in o/d top.

So 3800rpm should = 77.36mph.

Yes there can be some variation in total circumference from tyre to tyre... but his numbers are quite a significant way off so I suspect one if not both gauges are misreading... the addition of an o/d without the corresponding change in speedo and speedo gear on the box will also impact accuracy.

Bill Blue · May 25, 2016

Spinning #3 rod in a 1600 cc engine does not carry much weight. My Series 3, bought "new" with 127 miles on the odometer spun #3 at 15,000 miles. It had a slight "tick" in the engine when purchased. After #3 was replaced, standard insert was all that was needed, the "tick" disappeared. While I did not baby the car, I am not abusive. That is the only rod bearing I have ever had to replace. Sometimes I think they came from the factory that way.

Bill

Hodee · May 26, 2016

Thanks for the info guys. Tom H set my tach so I trust that is good, and I used a GPS to calculate speed. I get 75 mph at 4000 rpm so if I cruise between 3600-3800 that should be a safe pace at 45-50 psi oil pressure. If my valve seats or rod bearings go to crap then I'll rebuild. 62,000 (true original) miles and runs like a dream. Proud of all of you with project cars, but I'm happier to just go out and drive this thing on country roads 11-12 months out of the year. I consider myself spoiled and lucky. This site is wonderful.

Jimjordan2 · May 28, 2016

OK, so if all else is correct, engine, OD transmission, rear end size, size of tires, speedometer is reading correctly by my GPS, which is correct compared to my Lexus speedometer. Lexus? LOL.
Then I have to suspect my tachometer is incorrect. I think I have an old Dwell/Tachometer around here. Will pull it out and check some more.

Which brings us back to the original questions.

60-65 MPH feels like a real comfortable speed on the freeway. 70 is OK, I don't need to do more than that at this time, but it will go faster. Of course here in California, you get passed a lot doing 65. But by driving a Sunbeam we don't get dirty looks for going so slow.

Bill Blue · May 28, 2016

Be careful of any old electronic device. I had an interesting exchange a few years ago with an owner who insisted he was turning some fantastic rpm at 60 mph. He "knew" that the dash tach was correct because he had checked it against his (old) hand held tach. I finally told him that if he was turning those rpm's, he had a drive train problem. That prompted him to take it to a mechanic and found that while both his hand held and dash tach were way out of calibration, they were in total agreement.

Trust nothing. Not even me. On second thought, maybe I should say "especially me".

Bill

Tom H · May 28, 2016

Jim, If your tach has not been re-calibrated since it was built I can just about guarantee it is not accurate. I have redone over 3 dozen of these and on every one there are parts that have drifted so far from original value ( some over 100% shift) that accuracy is way off. These tachs are designed well, and they used highly stable parts in the few places it was needed, but most of the components were designed to be OK for maybe 10-20 years. I don't think the builders ever expected someone to be relying on the readings 40-plus years later!

Tom